Multi-conductor cables, such as telephone communications cables for example, are spliced together to serve a particular function. Each splice is a potential location where water may enter the cable, causing a deterioration in the service provided by the cable. Water presents a severe problem, not only for the splice joint but for cables in general. Steps taken to protect cables from moisture include pressurizing the cable with nitrogen or air to prevent water from entering. The conductors in a multiconductor cable do not completely fill the space within the cable sheath and the compressed gas occupies the spaces or interstices inside the cable sheath between the conductors. The pressurized gas escapes through any break occurring in the sheath, preventing water from entering. Obviously, a splice for a gas pressurized cable must be both gas tight and water tight.
Other cables, known as filled cables, have a filling compound such as a grease based or petroleum jelly to fill the interstices in the cable. Any water entering a filled cable must displace the fill. One advantage a gas pressurized cable has over a filled cable is that a break in the sheath is immediately detectable before water has a chance to enter, because of a drop in the pressure of the compressed gas. The pressure drop is easily detected with pressure monitoring equipment. Filled cables, on the other hand, have no early warning or detection mechanism and extra care must be taken to ensure that the splices are sealed from moisture. Obviously, a splice for a filled cable only needs to be water tight. Accordingly, cable splice closure methods and apparatus for filled cables are different from cable splice methods and apparatus for gas pressurized cables.
A grease filled cable assembly for protecting a cable splice against the environment is disclosed in U.S. Pat. No. 3,895,180. The cable splice assembly includes an inner cover and an outer jacket placed about the splice. The space between the inner cover and the outer jacket is filled with a liquid foam material which expands and hardens. The inner cover is injection filled with a grease based or petroleum jelly. U.S. Pat. No. 4,466,843 discloses a method for protecting a splice in a grease filled cable which includes forming a reservoir about the splice, filling the reservoir with a curable liquid sealant, and compressing the reservoir to force the liquid sealant to penetrate into the interstices between the individual wires of the splice bundle. The method also includes maintaining the pressure until the sealant cures, U.S. Pat. No. 4,511,415 discloses a method of sealing an electrical cable which includes forming a reservoir around the splice area, pouring an encapsulant into the reservoir and sealably covering the reservoir. While all of these cable splicing methods afford a certain level of protection against the entrance of water, none provides a level of protection commensurate with the current need to hold back a water pressure head of twenty feet or more.
A water pressure head, or simply water head, is the difference in elevation between two points in a body of water and the resulting pressure of the water at the lower point is expressible as this height. Thus, a 20-foot water head refers to the pressure exerted on the bottom of a column of water twenty feet high. Water is particularly troublesome for underground cables and buried cables because the cables and splices are exposed to water acting through a water head. Water finding its way to a break in a cable sheath typically acts through a water head of several feet and has the impetus to find its way to vulnerable splices.
Conventional splice closures were typically designed to protect the cable from a water head of approximately ten feet, which is two feet greater than the eight feet typically recommended by specifications. In practice, a pressure of 1 psi on the splice holds back a water head of two feet, so that a pressure of 6 psi is required to hold back a 12-foot water head. To achieve the 6 psi, some conventional splice closure methods covered the reservoir with a sealing tape applied in a stretched condition to pressurize the encapsulant in the splice area. The tape was applied without tools but had built-in distortion gauges giving an indication of the amount of stretch of the tape indicating pressure. The tape was often stretched, giving an indication of a specific pressure but inadvertently applied at a different pressure. Accordingly, it will be appreciated that it would be highly desirable to provide a cable splice apparatus and method for applying the tape at a uniform pressure.
Even when the tape was applied with a predetermined stretch developing a predetermined pressure on the encapsulant, the pressure dropped when the encapsulant cured. This pressure drop occurred because the uncured encapsulant was a pressure transmitting liquid whereas the cured encapsulant was a nonpressure transmitting solid. Measurements indicate the pressure dropped by about one half.
For the high pair-count cables typically encountered in communications networks, a closure is used to cover and protect the splice. The closure is filled with an encapsulant material. Such a closure requires a relatively large volume of expensive encapsulant. Accordingly, it is highly desirable to have a splice closure which uses a minimal amount of expensive encapsulant to protect the splice from the entrance of water.
Space is limited in most underground applications, especially in cable vaults. Splice closures have definite minimum space requirements depending upon the pair count and diameter of the cables involved. To accommodate cable retention clamps and to facilitate bending or twisting of the cables, the length of a splice closure is increased. It will be appreciated that it would be highly desirable to have a splice closure with the shortest possible length to reduce the minimum space requirement for a cable of a given diameter and pair count.